1. Field of the Invention
The present invention relates to a method by determination of the functional residual capacity (FRC) of lungs, and to a ventilator device for the determination of the functional residual capacity.
2. Description of the Prior Art
The functional residual capacity is the volume of gas remaining in the lungs and airways at the end of a normal expiration. This gas volume serves, inter alia, as a reservoir of oxygen for the body. A reduction in FRC affects a number of functions in the respiratory organs, such as airway resistance, lung compliance, the distribution of inspired gas and arterial oxygenation. It is therefore an advantage if FRC can be measured in a simple way as one step in diagnosing a patient. Measurement of FRC is also useful during anaesthesia, since the administration of an anaesthetic reduces FRC.
One way of measuring FRC is described in a doctoral thesis entitled "Measurement of Functional Residual Capacity by Sulphur Hexafluoride Washout" (pp. 11-15) by C. Jonmarker, Dept. of Anaesthesia and Clinical Physiology, University of Lund, Lund 1985. An inert, non-toxic trace gas (sulphur hexafluoride, SF.sub.6), is fed into the lungs of a patient during a wash-in phase. The gas is supplied directly to the lungs through a tracheal tube arranged after a gas meter. The gas meter measures the concentration of SF.sub.6 in gas expired from the lungs. After a number of respiratory cycles, an equilibrium will arise, in which the concentration of SF.sub.6 in the lungs is the same as the concentration of SF.sub.6 supplied in the inspired gas, e.g. 0.5%, which is less than the hygienic limit value for SF.sub.6. A washout phase, in which the administration of SF.sub.6 is stopped, is then begun, and the lungs are gradually emptied of SF.sub.6. During this washout phase, the concentration of SF.sub.6 in the expired gas and the flow of expired gas are measured. The washout phase continues until the concentration drops below a defined level, e.g. 0.01%. The total volume of SF.sub.6 washed out of the lungs is then calculated from the values measured for concentration and flow. A correction is made to compensate for the re-breathing of a certain amount of gas in the transition from expiration to inspiration. FRC is then calculated as the volume of SF.sub.6 divided by the concentration of SF.sub.6 at the end of the wash-in phase. This concentration is regarded as the concentration in the alveoli. During the actual measurement, the gas meter is periodically zeroed during the inspiratory phases. As a rule, the gas meter is calibrated with a test gas before the system is connected to a patient.
FRC can also be determined using helium, He, or nitrogen (N.sub.2), as the trace gas.
In the known method and equipment, a breathing gas is supplied by a ventilator, and the trace gas is supplied by a separate gas source. This means that a number of units must be interconnected when an FRC measurement is to be made. The separate supply of trace gas also means that the trace gas and the breathing gas might not mix completely before being fed into the lungs. Furthermore, the gas meter is calibrated with a test gas before the; equipment is set up. Disruptions thus could affect the calibration before measurements are made. A conventional IR gas meter is often used when SF.sub.6 is the trace gas. A cuvette is then irradiated with IR light, and absorption is measured at a specific wavelength. The meter is calibrated with clean cuvette windows, but the windows may have become slightly soiled at the time FRC measurement begins. This would affect the measurement and result in an erroneous reading.